Fouling of marine vessel hulls and other structures in a marine environment has always been a serious problem. The formation of incrustations of barnacles, tunicates, and like fouling organisms, will increase the vessel's weight, thereby decreasing the available storage space, slow a vessel underway, increase its fuel consumption, and make it difficult to handle, thus reducing the vessel's performance and efficiency. On fixed structures, fouling increases weight, and thus structural loading. Fouling also damages the vessel hull base paint, thereby exposing the hull to corrosion.
Vessel hull fouling can be removed while the vessel is in place or in dry-dock using mechanical and/or chemical means. However, these alternatives are frequently unavailable, or are available only after a long wait. When a vessel hull or structure is cleaned in place, it is common practice to use divers, however there are inherent dangers whenever a diver enters the water. Additionally, damage may occur whenever a diver cleans a hull or structure. When a vessel hull is cleaned in dry-dock, the vessel must be taken out of service to the nearest available dry-dock, which usually results in substantial adverse financial consequences due to the costs, not only for the required work, but also for the off-hire time. Furthermore, removal of incrustations of marine organisms while at dock can raise significant regulatory and environmental concerns. It is impractical to remove fixed structures from site for cleaning.
Remedies that have previously been tried include using toxic paints that slowly release such marine growth inhibitors such as copper or tin salts, or using silicone based paints, which are ultra-smooth, making it difficult for fouling organisms to adhere to the surface of the vessel hull. These methods are effective until the inhibitors are leached from the paint, or the paint is damaged, and fouling takes place again, requiring dry-docking of the vessel to remove the fouling material and to repaint the hull. Also, these anti-fouling agents remain in the marine environment for a long period of time. Therefore, the most toxic of the anti-fouling coatings are being banned worldwide and are being replaced by less toxic, but also less effective coatings. For structures and vessels expected to operate in a marine environment for a long period of time, such as FSOs or FPSOs, fouling is an even greater problem.
Another approach for controlling and preventing marine fouling involves using an anti-fouling system that includes a pair of electrodes positioned on opposite sides of the keel of a vessel, and a means for supplying an electrical current to the electrodes. The electrolysis of sea water produces toxic agents such as chlorine and sodium hypochlorite adjacent the vessel hull that remove barnacles, algae, fungi and other marine growths.
However, such systems do not provide predictable control of the concentration of anti-fouling composition delivered to the hull. In addition, the electrodes require regular maintenance, which may be difficult since the electrodes are positioned on the outside of the vessel hull adjacent the keel.